Added escaping for the trans block

Bug fix: yes
Feature addition: no
Backwards compatibility break: yes
Fixes the following tickets: #2713
Todo: see below
License of the code: MIT
Documentation PR: ~

This fixes an escaping problem with using the ` {% trans %} ` block which wasn't escaping the translations.

Imho since this is such a behavior change, it should be done in master branch. Also I'm not sure about it but I think I must add the change in both changelog and documentation.

There's one thing that I don't really like about this, the fact that I've hardcoded the 'html' string as a context but I don't know how to get the context either at compile time or in the compiled template. Any help would be great!